Gas to liquids processes that combine a reforming technology for production of synthesis gas with a Fischer-Tropsch process are well known. A variety of reforming technologies and Fischer-Tropsch reactor technologies are available and have differing efficiencies, complexities, scalabilities and costs. Three main technologies for the reforming of the synthesis gas are known and they are steam reforming, auto-thermal reforming and catalytic partial oxidation. For the largest scale processes the reforming technology of choice is usually auto thermal reforming as this produces the highest levels of thermal efficiency, operates with the lowest amount of steam and is the most straightforward for building in high capacity single trains for large world scale plants. This is typically combined with a slurry phase Fischer-Tropsch process utilising a cobalt catalyst. The description of the development of these technologies is well documented in texts such as A. P. Steynberg and M. E Dry, Fischer-Tropsch Technology, v 152, Studies in Surface Science and Catalysis, which is incorporated herein by reference.
While the drivers for world scale plants is to achieve competitive pricing through the construction of ever larger plants; the high levels of capital investment that are required for such large plants predicates that the plants must be built at large gas reserves capable of producing high rates of gas for many years: fields larger than 1 TcF.
However, much of the world's gas resources are contained within smaller widely separated fields where there is insufficient gas to provide a return on a large scale costly plant. In these circumstances the challenge is to produce a reduced cost plant that is optimised for manufacturing on a small scale with the minimum number of process units.
The concept of a simplified gas to liquids process has been discussed in a series of papers including “A new concept for the production of liquid hydrocarbons from natural gas in remote areas” by K Hedden, A. Jess and T Kuntze, Oil Gas—European Magazine 1994, which is incorporated herein by reference. Auto thermal reactors typically have used a burner wherein the light hydrocarbon stream is partially combusted with air or oxygen enriched air in combination with steam, whereby the thermal energy generated in the combustion produced a high temperature gas that is subsequently passed across a reforming catalyst within the same pressure vessel. This technology requires steam to be present that prevents the formation of soot and reduces catalyst deactivation. An alternate proposal seen in U.S. Pat. No. 6,344,491 is that the steam can be reduced or eliminated through carrying out the partial combustion step catalytically in a high velocity partial oxidation reactor. U.S. Pat. No. 6,344,491 is incorporated herein by reference. While the high velocity reactor can serve to reduce the size of the auto thermal reactor by no longer requiring an extended residence time for the combustion to complete and further reduces or eliminates the use of steam, it requires careful control to reduce the risk of an uncontrolled combustion. In a conventional auto thermal reforming the oxidant and hydrocarbon react as they mix, which allows for a safer reactor and allows higher levels of gas-preheating, however with catalytic partial combustion auto thermal reforming the safety of the system relies on the maintenance of high velocities within the reactor. Furthermore, with fixed bed pre-mixed partial oxidation it is found that catalyst overheating occurs such that the catalyst surface exceeds the temperature of the surrounding gas leading to increased catalyst deactivation. In addition temperatures within an air fed auto thermal or partial oxidation reactor that significantly exceed 1000° C. can lead to formation of nitrogen oxides that must be removed before a Fischer-Tropsch reactor so as to avoid catalyst poisoning.
A method of reforming methane for the production of synthesis gas is described in WO 2004/098750 and this syngas is described as suitable for conversion to Fischer-Tropsch products the patent does not offer a solution as to which process is most suitable for the production of the Fischer Tropsch products.
The use of fine particles or Fischer-Tropsch Catalyst to achieve high catalyst activity has been described such as in GB 2403481 utilising particles that range from 10 to 700 micron to achieve high catalyst activity, however these must be used within a liquid suspension to achieve efficient contact with the syngas and avoid excessive pressure drop. It is noted in this patent that the catalyst is ultimately removed through the use of porous filters but no reaction takes place on the filters in the absence of the syngas.
The presence of steam within the process gas which is utilised in conventional auto thermal reforming is detrimental to the simplification of the process that is desired for small scale production. The steam increases the thermal energy contained within the process gas that must be recovered to maintain the thermal efficiency. This results in a heat recovery system that is integral to maintaining thermal efficiency. However, the necessity to recover the heat increases the complexity, size and capital cost of the plant employed.
The challenge of building large Fischer-Tropsch reactors is well described by A. P. Steynberg and M. E Dry, Fischer-Tropsch Technology, v 152, Studies in Surface Science and Catalysis. For the largest scales the difficulties of producing tube sheets with diameters of several meters impacts on the cost and so part of the benefit for slurry bed technology comes from the ease of fabrication of the largest reactors. At the smaller levels of production the reactor complexity can be increased are relatively little incremental cost as the fabrication challenges are lower.
The challenges of heat transfer, mass transfer and volumetric efficiency for the Fischer-Tropsch reactor design is well described in the paper R. Guettel, T. Turek, Comparison of Different Reactor Types for Low Temperature Fischer-Tropsch Synthesis: A Simulation Study, Chemical Engineering Science, 64, (2009), 955-964, which illustrates the advantages and potential of the various technologies that are available for hydrocarbon liquid synthesis. While it is relatively straightforward to produce a cobalt catalyst for Fischer-Tropsch hydrocarbon production that can operate efficiently on the scale of a few grams of catalyst, this paper highlights the challenges of producing a reactor design capable of maintaining this performance at a commercial scale. Inherently a fixed bed of catalyst cannot operate with high cobalt efficiency unless particles of less than 200 microns are used. However, utilising small particles requires using low gas velocities if an excessive pressure drop is to be avoided. This results in poor heat transfer capabilities if the catalyst is simply packed within conventionally sized tubes of 25 mm diameter. The alternative would appear to be to coat the surface of a plate style reactor with particles of catalyst. While this solves the problem of the heat transfer and provides more heat transfer surface than is actually needed the construction methods of these types of reactor require that the process gas and catalyst occupies typically 40% or less of the total reactor volume. Taking into account the manifolding and any pressure containing shell that is required can result in a very low volumetric efficiency of catalyst packing and a high specific reactor capital cost. Some of this loss in efficiency can be recovered through operating the catalyst at higher temperature and with a higher inherent efficiency, but this can result in a reduced catalyst life and lower selectivities to desired hydrocarbon product.
One alternative proposed that allows a high activity bed to be developed is to use a structured catalyst such as described in Itenberg et al. US 2005/0032921 which utilises a high permeability cylindrical structure with a typical equivalent fixed bed depth of approximately 5 mm. US 2005/0032921 is incorporated herein by reference. The gas is forced through the porous structure which allows the catalyst to operate without severe mass transfer restrictions. The thermal conductivity of the fused catalyst structure is sufficient to avoid temperatures differentials of more than 5° C. building up across the membrane structure.
This goes some way to illustrate the method by which the cobalt structure can be incorporated within the reactor to maintain the cobalt catalyst efficiency but does not describe how the heat can best be removed from the reactor. It also restricts the catalyst formulation to one that can be fused to produce a support structure that is mechanically strong enough to be utilised within a commercial reactor. The problem of producing high mechanical strength catalysts that are capable of surviving either slurry phase attrition or the forces associated with the high pressure drops and packing stresses of a fixed bed process.
Another alternative is the use of slurry bed technology where the catalyst particle is suspended within liquid product mixture agitated by gas sparging which while delivering a reactor that has a higher volumetric loading of cobalt within the reactor and a high catalyst effectiveness through the use of small suspended particles suffers from the difficulties associated with catalyst attrition. The fine catalyst particles must be removed from the product solution utilising filtration, either internal or external to the reactor. These filters have a tendency to block as a result of the catalyst attrition inherent to the process.
What is needed is a reactor design that enables a high heat transfer solution to be placed within a Fischer-Tropsch reactor that enables a high catalyst efficiency to be maintained. It also requires a catalyst support structure that allows the formulations of cobalt catalyst that have high levels of reducibility and activity to be incorporated into the structure without the constraints of mechanical strength and thermal conductivity.
High heat transfer within a reactor can be achieved utilizing flat plate style of heat exchangers that do not encapsulate the catalyst for example US 2009/0145589A1 but this does not utilize a structured support for the catalyst to enable forced flow of reactant gas through the catalyst pore structure to enhance the catalyst activity.
Consequently there is a continuing search for a gas to liquids technology suitable for small scale operation that minimises capital cost through the use of technology requiring the minimal of integration, avoiding compression of the syngas between reformer and Fischer-Tropsch unit and providing a Fischer-Tropsch reactor capable of high conversion and operating with high catalyst activities.
It is an object of at least one aspect of the present invention to obviate or mitigate at least one or more of the aforementioned problems.
It is a further object of at least one aspect of the present invention to provide an improved Fischer-Tropsch process which avoids compression of syngas between reformer and a Fischer-Tropsch unit and provides a Fischer-Tropsch reactor capable of high conversion and operating with high catalyst activities.